Dual stack optical data storage medium and use of such medium

ABSTRACT

A dual-stack optical data storage medium ( 20 ) is described for read out using a focused radiation beam ( 29 ) with a wavelength of 400-410 nm and a Numerical Aperture (NA) of 0.84-0.86. The medium has a substrate ( 21 ) and a first stack of layers named L 0  ( 22 ) comprising a first information layer and a second stack of layers named L 1  ( 23 ), comprising a second information layer. A radiation beam ( 29 ) transparent spacer layer ( 24 ) is present between L 0  and L 1.  A transmission stack named TS 0  with a thickness d TS0  and an effective refractive index n TS0  contains all layers between L 0  and an entrance face ( 26 ) of the medium ( 20 ). A transmission stack named TS 1  with a thickness d TS1  and an effective refractive index n TS1  containing all layers between L 1  and the entrance face ( 26 ). The spacer layer ( 24 ) has a thickness selected from the range 20-30 μm, the thickness d TS0  in dependence on the refractive index n TS0  and the thickness d TS1  in dependence on the refractive index n TS0  are within a specified area. In this way a reliable read out of both the first and the second information layer of respectively L 0  and L 1  is achieved.

The invention relates to a dual-stack optical data storage medium for atleast read out using a focused radiation beam with a wavelength λbetween 400 nm and 410 nm and an Numerical Aperture (NA) between 0.84and 0.86, entering through an entrance face of the medium during readout, comprising:

-   -   a substrate with present on a side thereof:    -   a first stack of layers named L0, comprising a first information        layer,    -   a second stack of layers named L1, comprising a second        information layer, L1 being present at a position closest to the        entrance face and L0 more remote from the entrance face than L1,    -   a radiation beam transparent spacer layer between L0 and L1,    -   a radiation beam transparent cover layer between the entrance        face and L1    -   a transmission stack named TS0 with a thickness d_(TS0) and an        effective refractive index n_(TS0) containing all layers between        L0 and the entrance face,    -   a transmission stack named TS1 with a thickness d_(TS1) and an        effective refractive index n_(TS1) containing all layers between        L1 and the entrance face.

The invention also relates to the use of such medium.

An embodiment of such an optical recording medium is known from a paper“New Replication Process Using Function-assigned Resins for Dual-layeredDisc with 0.1 mm thick Cover layer”, by K. Hayashi, K. Hisada and E.Ohno, Technical Digest ISOM 2001, Taipei, Taiwan. A minimum spacer layerthickness of 30 μm was disclosed.

There is a constant drive for obtaining optical storage media suitablefor recording and reproducing, which have a storage capacity of 8Gigabyte (GB) or larger. This requirement is met by some Digital VideoDisk or sometimes also Digital Versatile Disk formats (DVD). DVD formatscan be divided into DVD-ROM that is exclusively for reproduction,DVD-RAM, DVD-RW and DVD+RW, which are also usable for rewritable datastorage, and DVD-R, which is recordable once. Presently the DVD formatscomprise disks with capacities of 4.7 GB, 8.5 GB, 9.4 GB and 17 GB.

The 8.5 GB and, in particular, the 9.4 GB (DVD-9) and 17 GB (DVD-18)formats exhibit more complicated constructions and usually comprisemultiple information storage layers. The 4.7 GB single layer re-writableDVD format is easy to handle comparable, for example, to a conventionalcompact disk (CD) but offers an insufficient storage capacity for videorecording purposes.

A high storage capacity format that recently has been suggested isDigital Video Recording (DVR). Two formats are currently beingdeveloped: DVR-red and DVR-blue, the latter also called Blu-ray Disc(BD), where red and blue refer to the used radiation beam wavelength forrecording and reading. This disk overcomes the capacity problem and, inits simplest form, has a single storage layer format which is suitablefor high density digital video recording and storage having a capacityabove 22 GB in the DVR-blue format.

The DVR disk generally comprises a disk-shaped substrate exhibiting onone or both surfaces an information storage layer. The DVR disk furthercomprises one or more radiation beam transmissive layers. These layersare transmissive to the radiation beam that is used to read from orwrite into the disk. For example a transmissive cover layer, which isapplied on the information storage layer. Generally, for high-densitydisks, lenses with high numerical aperture (NA), e.g. higher than 0.60,are used for focusing such a radiation beam with a relatively lowwavelength. For systems with NA's above 0.60 it becomes increasinglydifficult to apply substrate incident recording with substratethicknesses in the 0.6-1.2 mm range due to decreasing tolerances on e.g.thickness variations and disk tilt. For this reason, when using disksthat are recorded and read out with a high NA, focusing onto a recordinglayer of a first recording stack, is performed from the side oppositefrom the substrate. Because the first recording layer has to beprotected from the environment at least one relatively thin radiationbeam transmissive cover layer, e.g. thinner than 0.5 mm, is used throughwhich the radiation beam is focused. Clearly the need for the substrateto be radiation beam transmissive no longer exists and other substratematerials, e.g. metals or alloys thereof, may be used.

A dual-stack optical storage medium has two reflective informationlayers, that are read-out from the same side of the medium. In this dualstack medium case, where a second recording stack is present, aradiation beam transmissive spacer layer is required between therecording stacks. The first recording stack must be at least partiallytransparent to the radiation beam wavelength in order to make readingfrom the recording layer of the second recording stack possible. Thethickness of such spacer layers typically is thicker than 30 μm. Theradiation beam transmissive layer or layers which are present betweenthe radiation beam source and the recording stack that is most remotefrom the substrate are normally called cover layers. When prefabricatedsheets are used as transmissive layers extra transmissive adhesivelayers are required in order to bond cover layers to each other.

In the DVR disk the variation or unevenness of the thickness of theradiation beam transmissive layers over the radial extension of the diskhas to be controlled very carefully in order to minimize the variationin the optical path length for the impinging radiation. Especially theoptical quality of the radiation beam at the focal point in the BD orDVR-blue version, which uses a radiation beam with a wavelengthsubstantially equal to 405 nm and an NA substantially equal to 0.85, isrelatively sensitive to variations in the thickness of the transmissivelayers. The total layer thickness has an optimal value in order toobtain minimum optical spherical aberration of the focused radiationbeam on, e.g., the first information recording layer. A deviation, e.g.±5 μm, from this optimal thickness already introduces a considerableamount of this kind of aberration. Because of this small range it isimportant that the average thickness of the transmissive layers is equalto or close to its optimal thickness in order to make optimal use of thetolerances of the system and to have a high yield in manufacturing themedium. Assuming that a thickness error is Gaussian distributed aroundthe nominal setting of the thickness, it is clear that the number ofmanufactured disks which do not comply with the above specification isminimal when the target setting of the nominal thickness duringmanufacture is substantially equal to the optimal thickness of the coverlayer as in the specification of the DVR disk. Two studies of thespacer-layer thickness were published recently for DVD dual-layer discs.A numerical aperture of 0.6, readout through the substrate of 0.58 mmand light of 405 nm wavelength were used. An optimum spacer layerthickness of 30 um has been found by Lee et al. Jpn. J. Appl. Phys. Vol40 (2001) pp 1643-1644 and 40 μm was found by Higuchi and Koyanagi, Jpn.J. Appl. Phys. Vol. 39 (2000) 933 [4].

For a system with 0.1 mm thin cover layer and a high NA of 0.85 and awavelength of 405 nm additional correction of the spherical aberration(proportional to λ/NA⁴) is required. To neglect the interference fromthe neighboring layer a spacer layer of minimally 30 μm has beenconsidered necessary. This has the disadvantage that the drive designfor reading out such a medium in such case has to be rather complicatedin order to cover the necessary range for spherical aberrationcorrection. Further the cover layer of such medium may become relativelythin and the underlying layers are more susceptible to damage.

It is an object of the invention to provide a medium of the kind asdescribed in the opening paragraph with a reliable read out of data fromthe first information layer and form the second information layer.

This object is achieved in accordance with the invention by an opticaldata storage medium which is characterized in that the spacer layer hasa thickness selected from the range 20-30 μm, the thickness d_(TS0) independence on the refractive index n_(TS0) is within the upper shadedarea in FIG. 1 and the thickness d_(TS1) in dependence on the refractiveindex n_(TS1) is within the lower shaded area in FIG. 1. Thespecifications of the Transmission Stacks (TS) include all possiblelayers on top of the concerning recording stack, such as e.g. gluinglayers in case of foils, the spacer layer and the semi-transparentrecording stack of L1 in case of TS0, the Cover Layer and possibly aProtective coating). From EP-A-1047055 it is known to use a polymerlayer such as, for example, a polycarbonate (PC) sheet aslight-transmissive cover or spacer layer and adhere such layer to theinformation storage layer by means of a thin, spin-coated layer of a UVcurable liquid resin or a pressure sensitive adhesive (PSA).

To find the minimal spacer-layer thickness for the blue system with highNA the dependence of the data quality was studied when read out from themedium as function of the spacer-layer thickness. It was found that ingeneral, the spacer layer thickness or the amount of separation of thefirst information and the second information layer depends on the sizeof the photo-detector in the optical pick-up unit (OPU) of the opticalmedium drive, the magnification from the photo-detector to the medium,the reflectivity ratio of the first and second information layers andthe distance between the two layers, i.e. the thickness of the spacerlayer. A stable OPU design restricts the size of the photo detector andthe magnification of objective lens and collimator lens. Tolerance foraging and alignment errors require a minimum detector size of 100 μm anda magnification of about 10. First the influence of the stray-light onthe recording performance has been modeled. The main influence comesfrom reduction of the signal modulation resulting in a decrease of thesignal to noise ratio (FIG. 3). In a second step, the amount ofstray-light as function of the spacer-layer thickness is simulated usingray tracing (FIG. 4).

In an embodiment the maximum deviations of d_(TS0) and d_(TS1) fromrespectively the average values of d_(TS0) and d_(TS1) between a radiusof 23 mm and 24 mm of the medium do not exceed ±2 μm measured over thewhole area of the medium. This has the advantage that no substantialcorrection for spherical aberration is required when the firstinformation layer of the medium or the second information layer of themedium is scanned by the optical medium drive. During scanning the OPUwill move radially inward our radially outward while the medium rotates.When the thickness variations of TS0 and TS1 are within said limits alsothe spherical aberration stays within acceptable limits over the wholearea of the medium. The only instance when correction is required iswhen the OPU switches from focusing onto the first information layer tofocusing onto the second information layer or vice versa.

In another embodiment n_(TS0) and n_(TS1) both have a value of 1.6 andthe following conditions are fullfilled: 95 μm≦d_(TS0)≦105 μm and 70μm≦d_(TS1)≦80 μm. Most plastic materials used as transparent layers havea refractive index of 1.6 or substantially close hereto. In this casereliable read out is possible when the thicknesses fall within thementioned ranges.

In a further embodiment the spacer layer thickness is 25 μm orsubstantially close to 25 μm and the cover layer thickness is 75 μm orsubstantially close to 75 μm. It is advantageous from a viewpoint ofmanufacture to use a substantial fixed value of the spacer and coverlayer thickness. For instance, one method of manufacture comprises theapplication of a pressure sensitive adhesive (PSA) with a predeterminedthickness which is UV-cured after being brought in contact with otherlayers of the medium. This material is usually supplied as a sheet offoil with the PSA on one or sides and those sheets are made with apredetermined thickness.

The invention will be elucidated in greater detail with reference to theaccompanying drawings in which

FIG. 1 shows the allowable area of thickness of the transmission stacksTS0 and TS1 as a function of the refractive index.

FIG. 2 schematically shows the layout of a dual-stack recording mediumaccording to the invention.

FIG. 3 shows a simulation of data-to-clock jitter when read out asfunction of the stray-light from the adjacent, out-of-focus, informationlayer.

FIG. 4 shows a ray-tracing simulation of the light reflected onto thephoto-detector as function of the spacer-layer thickness.

In FIG. 1 the allowed thickness ranges of TS0 and TS1 are indicated. Thethickness d_(TS0) in dependence on the refractive index n_(TS0) iswithin the upper shaded area 1 and the thickness d_(TS1) in dependenceon the refractive index n_(TS1) is within the lower shaded area 2. Thespacer layer 24 (FIG. 2) has a thickness selected from the range 20-30μm.

In FIG. 2 an embodiment of the dual-stack optical data storage medium 20according to the invention is shown. A focused laser beam 29 with awavelength λ of 405 nm and an Numerical Aperture (NA) of 0.85 entersthrough entrance face 26 of the medium 20 during read out. A substrate21 made of polycarbonate has present on a side thereof: a first stack oflayers 22 named L0 comprising a first information layer, a second stackof layers 23 named L1, comprising a second information layer. L1 ispresent at a position closest to the entrance face 26 and L0 is presentmore remote from the entrance face 26 than L1. A transparent spacerlayer 24 made of a UV cured resin, e.g. SD 694 made by DIC, is presentbetween L0 and L1. A transparent cover layer 25 is present between theentrance face 26 and L1 and may be made of the same material or a sheetof PC or PMMA with a pressure sensitive adhesive USA). The spacer layermay also be a sheet combined with PSA. The transmission stack named TS0has a thickness d_(TS0) of 100 μm and an effective refractive indexn_(TS0)=1.6 and contains all layers between L0 and the entrance face 26.The L1 stack 23 has a relatively low thickness of a maimally a fewhundred nm the influence of which may be neglected. Naturally L1 doesaffect the optical transmission but this aspect is not dealt with here.The transmission stack named TS1 has a thickness d_(TS1) of 75 μm and aneffective refractive index n_(TS1) of 1.6 and contains all layersbetween L1 and the entrance face (26). The spacer layer (24) has athickness of 25 μm. The thickness d_(TS0)=100 μm at a refractive indexn_(TS0)=1.6 falls within the upper shaded area in FIG. 1 and thethickness d_(TS1)=75 μm at a refractive index n_(TS0)=1.6 falls withinthe lower shaded area in FIG. 1.

In FIG. 3 the modeled data-to-clock jitter in %, when reading the firstinformation layer of L0, as function of the stray-light from the out offocus layer, e.g. the second information layer of L1, is represented bygraph 30. The jitter without stray-light was chosen to be 5.8%. At astray-light level of 15% the jitter has increased from 5.8% to 6.5%which is tolerable.

In FIG. 4 the ray-tracing simulation of the light reflected onto thephoto-detector as function of the spacer-layer thickness is representedby graph 40. A 15% upper limit on the stray-light is represented bydotted line 41. The stray-light as function of the spacer-layerthickness was calculated for a OPU detector size of 100 μm and amagnification factor of 10. The minimum spacer layer 24 (FIG. 2)thickness to guarantee less than 15% stray-light is 20 μm.

According to the invention a dual-stack optical data storage medium isdescribed for read out using a focused radiation beam with a wavelengthof 400-410 nm and a Numerical Aperture (NA) of 0.84-0.86. The medium hasa substrate and a first stack of layers named L0 comprising a firstinformation layer and a second stack of layers named L1, comprising asecond information layer. A radiation beam transparent spacer layer ispresent between L0 and L1. A transmission stack named TS0 with athickness d_(TS0) and an effective refractive index n_(TS0) contains alllayers between L0 and an entrance face of the medium. A transmissionstack named TS1 with a thickness d_(TS1) and an effective refractiveindex n_(TS1) containing all layers between L1 and the entrance face.The spacer layer has a thickness selected from the range 20-30 μm, thethickness d_(TS0) in dependence on the refractive index n_(TS0) and thethickness d_(TS1) in dependence on the refractive index n_(TS0) arewithin a specified area. In this way a reliable read out of both thefirst and the second information layer of respectively L0 and L1 isachieved.

1. A dual-stack optical data storage medium for at least read out usinga focused radiation beam with a wavelength λ between 400 nm and 410 nmand an Numerical Aperture (NA) between 0.84 and 0.86, entering throughan entrance face of the medium during read out, comprising: a substratewith present on a side thereof: a first stack of layers named L0comprising a first information layer, a second stack of layers named L1,comprising a second information layer, L1 being present at a positionclosest to the entrance face and L0 more remote from the entrance facethan L1, a radiation beam transparent spacer layer between L0 and L1, aradiation beam transparent cover layer between the entrance face and L1a transmission stack named TS0 with a thickness d_(TS0) and an effectiverefractive index n_(TS0) containing all layers between L0 and theentrance face, a transmission stack named TS1 with a thickness d_(TS1)and an effective refractive index n_(TS1) containing all layers betweenL1 and the entrance face, characterized in that the spacer layer has athickness selected from the range 20-30 μm, the thickness d_(TS0) independence on the refractive index n_(TS0) is within the upper shadedarea in FIG. 1 and the thickness d_(TS1) in dependence on the refractiveindex n_(TS0) is within the lower shaded area in FIG.
 1. 2. An opticaldata storage medium according to claim 1, wherein the maximum deviationsof d_(TS0) and d_(TS1) from respectively the average values of d_(TS0)and d_(TS1) between a radius of 23 mm and 24 mm of the medium do notexceed ±2 μm measured over the whole area of the medium.
 3. An opticaldata storage medium according to claim 1, wherein n_(TS0) and n_(TS1)both have a value of 1.6 and the following conditions are fullfilled: 95μm≦d_(TS0)≦105 μm and 70 μm≦d_(TS1)≦80 μm.
 4. An optical data storagemedium according to claim 1, wherein the spacer layer thickness is 25 μmor substantially close to 25 μm and the cover layer thickness is 75 μmor substantially close to 75 μm.
 5. Use of an optical data storagemedium as claimed in claim 1 for reliable data read out from both thefirst information layer and the second information layer.